This is the mistake I made: Both the times I sent it to the address below. but the first time, I picked USPS as a carrier. Below is the address they have stated if u pick any non usps carrier. Realized my mistake and called uscis customer service, they had no record of any documents received. The 2nd time around i sent it to the same address but via Fedex.within a week i received receipt notice.
I received the returned package(my 1st package) after my second shipment.

131 S Dearborn - 3rd floor
Chicago IL 60603-5517

hope this helps